Input: sysrq reset sequence

A simple binding to represent a set of keys as described in
include/uapi/linux/input.h. This is to communicate a sequence of keys to the
sysrq driver. Upon holding the keys for a specified amount of time (if
specified) the system is sync'ed and reset.

Key sequences are global to the system but all the keys in a set must be coming
from the same input device.

The /chosen node should contain a 'linux,sysrq-reset-seq' child node to define
a set of keys.

Required property:
keyset: array of Linux keycodes, one keycode per cell.

Optional property:
timeout-ms: duration keys must be pressed together in milliseconds before
generating a sysrq. If omitted the system is rebooted immediately when a valid
sequence has been recognized.

Example:

 chosen {
                linux,sysrq-reset-seq {
                        keyset = <0x03
                                  0x04
                                  0x0a>;
                        timeout-ms = <3000>;
                };
         };

Would represent KEY_2, KEY_3 and KEY_9.